1. Field of Invention
This invention relates to oral hygiene and therapeutic devices, especially to ultrasound oral hygiene and therapeutic devices.
2. Description of Prior Art
Presently the most popular personal dental cleaning device is still the traditional manual toothbrush. Although it has a long history and has been constantly improved, manual toothbrush suffers from a number of disadvantages:
1. It produces uneven result, more brushing on prominent and easy-to-reach areas and less brushing on depressed and hard-to-reach areas.
2. It does not clean areas that are inaccessible to brush bristles such as inter-dental and gingival areas.
3. It inherently involves a low brushing frequency.
4. It cleans a small area at a time, hence, it is time consuming and a user has to brush over the entire dental surface.
5. It is difficult to provide just the right amount of brushing.
6. It is difficult to brush along the direction of tooth crevices as recommended.
7. It is difficult to maintain just the right pressure.
8. It requires some dexterity from users, which is not typically the case for children or many handicapped adults.
9. It produces inconsistent results for different users and at different times for the same user.
10. The bristles and the abrasive particles in dentifrice can damage the dental surface and the gums if not used correctly.
11. Users have to move their hands vigorously, and such motion can cause muscle fatigue and stress.
12. The bristle tips wear out, hence, the cleaning results deteriorate quickly and the toothbrush has to be replaced regularly.
13. Other supplemental cleaning devices, such as dental floss, are needed.
The major challenge for dental cleaning is the imperfection and randomness of dental surface. The present remedy for this problem is to use bristles of different lengths, as suggested in U.S. Pat. No. 6,202,241 to Hassell et al. (2001) and U.S. Pat. No. 4,894,880 to Aznavoorian (1990) for example. However, since the contours of dental surfaces are so diverse from one individual to another, and from one area to another for the same individual, the result is hardly satisfactory. From this point of view alone, a smaller brush head works better. But a smaller brush head would worsen several other disadvantages listed above, especially disadvantage number 4. Some “whole-mouth” toothbrushes and “U”-shaped three-head toothbrushes have been proposed in attempt to remedy disadvantage number 4. For example, U.S. Pat. No. 4,237,574 to Kelly et al. (1980), U.S. Pat. No. 4,223,417 to Solow (1980), and U.S. Pat. No. 4,795,347 to Maurer (1989). Unfortunately, these designs do not provide satisfactory results as hoped and are difficult to use.
Another major challenge for dental cleaning is that there are areas inaccessible to bristles, such as inter-dental and gingival areas. Presently, supplemental cleaning devices, such as dental floss, toothpicks or the like, are required for cleaning the inter-dental areas. However, these devices can only be used in places that are directly accessible from the outside and are not effective in removing plaque. Furthermore, a user has to work in each tooth crevice, which is time and labor consuming. Another supplemental device for cleaning inter-dental and gingival areas is the high-pressure water jet. Examples of patents include U.S. Pat. No. 3,227,158 to Mattingly (1966) and U.S. Pat. No. 3,522,801 to Robinson (1970). Similar to dental floss, using a high-pressure water jet device is also time and labor consuming, and it is not very effective in removing plaque. In addition, high-pressure water jet irritates gums and may cause bleeding and damage.
Although the recommended brushing time is two minutes, people on average spend less than one minute. To control the time of a brushing session, it has been suggested to equip a timer on a toothbrush, for example, U.S. Pat. No. 6,106,294 to Daniel (2000) and U.S. Pat. No. 5,894,453 to Pond (1999). But a timer can only control the total time of a brushing session. Unless the user can uniformly distribute the preset total time, a timer is not very effective.
In the last few years, the electrical toothbrush began to gain popularity. The dominant group of electrical toothbrushes, which will be referred as ordinary electrical toothbrushes, consists of an electrical motor that produce rotational or vibrational motion, a brush head with bristles, and a mechanical transmission system to transmit the motion of the electrical motor to the brush head, while converting the original motion type of the electrical motor to the final motion type of the brush head.
In order to obtain the desired motion of the brush head, most of the prior-art designs have used rather complicated mechanical transmission systems involving many moving parts, especially those with complex motion type or with a multi-sectional brush head. Referring to a typical ordinary electrical toothbrush that is currently being sold on the market as an example, there is a transmission system that firstly converts the rotational motion of the electrical motor into a longitudinal back and forth motion of a driving shaft, and then converts this longitudinal back and forth motion into a reciprocal rotation of the brush head about an axis perpendicular to the shaft. Complex mechanical systems are inherently unreliable, costly, energy inefficient, and produce discomforting noise. There are hundreds of patents on ordinary electrical toothbrushes and several different types are being sold on the market currently.
The following are some examples of patents categorized by brush head motion types. U.S. Pat. No. 5,699,575 to Peifer (1997) and U.S. Pat. No. 5,146,642 to Mank et al. (1992) provide designs where the brush head rotates about the longitudinal axis of the brush shaft. U.S. Pat. No. 5,836,030 to Hazeu et al. (1998) and U.S. Pat. No. 5,383,242 to Bigler et al. (1995) provide designs where the circular brush head rotate reciprocally about its central axis. U.S. Pat. No. 5,934,908 to Woog et al. (1999) provides a design where the brush head rotates reciprocally about the longitudinal axis. U.S. Pat. No. 5,077,855 to Ambasz (1992) provides a design where the brush head vibrates along the longitudinal axis. U.S. Pat. No. 6,453,498 to Wu (2002) and U.S. Pat. No. 5,378,153 to Giuliani et al. (1995) provide designs where the brush head vibrates sideways. U.S. Pat. No. 4,336,622 to Teague Jr. et al. (1982), U.S. Pat. No. 4,787,847 to Martin et al. (1988), and U.S. Pat. No. 3,535,726 to Sawyer (1970) provide designs where the brush head strokes up and down. U.S. Pat. No. 5,974,615 to Schwarz-Hartmann et al. (1999) provides a design where the circular brush head rotates reciprocally about its central axis, as well as strokes up and down. U.S. Pat. No. 5,353,460 to Bauman (1994) and U.S. Pat. No. 5,524,312 to Tan et al. (1996) provide designs where the brush head has two sections, each of them having its own motion type.
Each of these particular motion types may be suitable for particular sections of dental surface, but none of them alone provides satisfactory results for the entire dental surface. The combined motion or multi-sectional designs may produce better results than single motion types. However, similarly to using bristles of different lengths, the improvement is rather limited. Some of the prior-art designs can detect excessive pressure exerted by the user and either send a warning signal or shut down the power. This provides some guidance and assistance, but it still relies on the user to make the necessary adjustments to maintain the right pressure.
Compared to manual toothbrushes, the most significant improvement that ordinary electrical toothbrushes bring about is the much greater brushing frequency. The fastest model currently sold on the market has a frequency of 500 Hz. This substantially overcomes the low brushing frequency disadvantage (number 3) of the manual toothbrush.
There are also powered versions of the whole-mouth toothbrushes or multi-head toothbrushes. Such as U.S. Pat. No. 5,177,827 to Ellison (1993), U.S. Pat. No. 4,224,710 to Solow (1980), U.S. Pat. No. 6,209,164 to Sato (2001), and U.S. Pat. No. 6,138,310 to Porper et al. (2000). Like their manual counterparts, they are not effective and rather difficult to use. Furthermore, they require very complex mechanical systems.
If all the best parts of the prior-art designs of the ordinary electrical toothbrush were combined, the result would overcome the disadvantages number 3 and 11, and improve to some extent on disadvantages number 1, 5, 6, 7 and 8. All the rest disadvantages of manual toothbrushes (2, 4, 9, 10, 12, and 13) would largely remain unresolved. Furthermore, the ordinary electrical toothbrushes suffer from a number of additional disadvantages of their own:
1. They consist of many moving mechanical parts, which often result in low reliability and high manufacture cost.
2. Brush heads are rather expensive, and they involve substantial cost in long run.
3. They produce discomforting noise.
4. Their mechanical transmission systems are energy inefficient, and the moving parts suffer from considerable friction, since they constantly undergo a stop-and-go (or forward-and-reverse) type of motion.
Ordinary electrical toothbrushes sometimes are referred as sonic electrical toothbrushes. It should be made clear that this simply means that they operate in the audible frequency range. The patents and product commercials of some ordinary electrical toothbrushes claim to have a “sonic wave” effect or even a “cavitation” effect, supposedly capable of cleaning areas that cannot be reached by the bristles, such as inter-dental and gingival areas. That should be more carefully considered, because, according to basic physical principles, ordinary electrical toothbrushes can hardly produce a sonic wave with adequate cleaning power, let alone any cavitation effect.
Acoustic waves in a liquid consist of alternating local compressions and dilatations. First of all, the brush bristle is very inefficient in converting mechanical vibration energy into acoustic wave energy, since it is too weak to significantly compress the liquid. Secondly, in typical tooth brushing there is only a thin layer of loose liquid bounded only by its surface tension. A sufficient degree of compression, or, more technically, acoustic pressure can hardly be produced in such a thin layer of loose liquid at low frequencies (e.g., 500 Hz), since the liquid by and large escapes compression by moving sideways. Thirdly, the bristles stir air into the thin layer of liquid making it into a foam form, which is a very poor acoustic medium that can hardly sustain significant acoustic pressure. Fourthly, the air bubbles and abrasive particles are strong acoustic scatters. The great population of these scatters in the liquid results in a large effective absorption coefficient that stops the propagation of acoustic wave within a very short distance. Fifthly, only longitudinal acoustic wave (propagating along the direction of vibration) can be supported in liquid. Based on their transverse motion, brush bristles can hardly generate longitudinal acoustic wave with sufficient cleaning power that propagates into inter-dental areas. Finally, the transmission of a wave through an opening is appreciable only when the smallest dimension of the opening is comparable to or larger than the wavelength of the wave. Since the wavelength of acoustic wave of 500 Hz is in the order of meters and the smallest dimension of the typical opening of inter-dental and gingival areas is in the order of millimeters, the acoustic wave is almost totally repelled from inter-dental and gingival areas.
Cavitation is created by much greater high and low pressure in the liquid, and that is the phenomenon on which ultrasound cleaning actually based. Here, the term cavitation refers to the vaporous cavitation rather than the gaseous cavitation that is associated with the dissolved air bubbles. Gaseous cavitation involves much lower acoustic intensity and hardly has any cleaning effect. In vaporous cavitation, small vapor bubbles are generated in the liquid by the low-pressure troughs. When the following high-pressure crests come along, these vapor bubbles implode, i.e., collapse rapidly. The nearby liquid rushes in at high speed to fill the space formerly occupied by the vapor bubble. This action results in a violent local agitation of the liquid, which produces a thorough cleaning effect. Among other conditions, cavitation requires a threshold of acoustic wave intensity of the order of a few watts per square centimeter. Such threshold is far beyond the limit of the combination of the thin layer of loose fluid and brush bristles. Furthermore, the diameter D of the vapor bubble is inversely proportional to the wave frequency f, according to the equation D=η/f , where η=600 cm/s. Large bubbles require even more stringent laboratory conditions. In practical conditions, cavitations mainly occur at ultrasonic frequencies, from 20 kHz to 1 MHz. Ordinary electrical toothbrushes, limited as they are by their mechanical systems, do not operate at such high frequencies at all.
In conclusion, the cleaning effect of ordinary electrical toothbrushes derives by and large from the ordinary abrasive brushing action. The corresponding acoustic-wave action is of little significance and there is no cavitation effect. Whatever superior cleaning effect over that of manual toothbrushes derives almost exclusively from the much greater frequency of the bristles motion.
Another group of proposed electrical toothbrushes in the prior art comprises the so-called ultrasonic toothbrushes. The brush heads of the ultrasonic toothbrushes are driven by ultrasonic transducers, rather than by electrical motors. See, for example, U.S. Pat. No. 5,546,624 to Bock (1996), U.S. Pat. No. 5,311,632 to Center (1994), U.S. Pat. No. 4,991,249 to Suroff (1991), and U.S. Pat. No. 4,333,197 to Kuris (1982). The basic idea behind the proposed ultrasonic toothbrushes is to make the brush head vibrate at ultrasonic frequencies, hoping that this could provide superior results to those of ordinary electrical toothbrushes. Unfortunately, the basic physical principles do not support that idea and none of the proposed ultrasonic toothbrush has been successfully produced.
Nonetheless, compared to any mechanical scrubbing mechanism, ultrasounds not only provide superior cleaning results, but also prevent damage to the object being cleaned, provided that appropriate intensity and duration are applied. Ultrasound cleaning can be used for extremely tough jobs such as cleaning a carbonized fuel injector, as well as extremely delicate jobs such as cleaning a semiconductor wafer. A very important property of ultrasound is that it can penetrate into small holes because of short wavelength. Hence, it can clean places that are inaccessible to ordinary mechanical cleaning tools. Because of this property, a mechanical assembly (such as the core of a mechanical watch) can be cleaned by ultrasound without having to be disassembled. So, ultrasound may indeed be an ideal means to clean teeth as a result of these properties, but only if used correctly. Otherwise, ultrasound is not only ineffective, but also hazardous.
Industrial ultrasound cleaning uses frequencies from 20 kHz up to several hundred kHz, and uses power levels from a few watts per gallon (of cleaning solution) up to a few hundred watts per gallon. When the ultrasound power level exceeds the cavitation threshold, millions of vapor bubbles are generated and subsequently implode in the cleaning solution during each wave cycle For an ultrasound of 100 kHz, there are one hundred thousand wave cycles in one second and the diameter of a vapor bubble is about 0.06 mm. These vapor bubbles, like millions of tiny brushes, work simultaneously on the entire surface of the object being cleaned.
Various ultrasound dental cleaning devices have been proposed. Although different in designing details, they all basically derive from other ultrasonic tools, such as ultrasonic drills or scalers, with modified applicators. Examples of the prior-art ultrasound dental cleaning devices include U.S. Pat. No. 6,514,077 to Wilk (2003), U.S. Pat. No. 4,176,454 to Hatter et al. (1979), U.S. Pat. No. 4,148,309 to Reibel (1979), and U.S. Pat. No. 4,071,956 to Andress (1978). Unfortunately, these prior-art designs disregard basic physical principles and safety issues of ultrasound application. In particular, they suffer from the following disadvantages.
1. They do not direct the ultrasound radiation towards the teeth (most of them in fact mainly direct ultrasound radiation towards the throat of the user).
2. The radiation surfaces of the applicators are too far from the teeth.
3. They provide either insufficient ultrasound radiation to the dental surface or excessive ultrasound radiation to other surfaces of the oral cavity.
4. They do not provide uniform ultrasound radiation on the dental surface.
5. Most of them operate with open mouth hence the upper teeth are not effectively cleaned or not cleaned at all.
6. They provide insufficient protection or no protection at all to mouth tissue.
7. They require large amounts of cleaning solution.
8. They are not energy efficient because of the nondiscriminatory radiation.
9. They are difficult to use, especially for self-use.
10. They involve high risk of injury and damage by ultrasound radiation.
In conclusion, despite the improvements of dental hygiene devices over the years, many problems remain either unsolved or their solutions remain unsatisfactory. There is definitely a need for a dental hygiene device that can substantially overcome the aforementioned disadvantages of the prior-art dental cleaning devices and provide overall satisfactory results.